In order to supply an electrical load (apparatus, load element), in particular an electrical drive or motor, with electrical energy or power, the load is generally connected to a line branch of an electrical supply network via a switching apparatus (a switching device) for switching the electric current on and off.
For switching electric currents, there are known, not only the predominantly used mechanical switching apparatuses with switching contacts but also electronic switching apparatuses realized with semiconductor components. Such semiconductor components can be subdivided into current controlled semiconductor components, including bipolar transistors and thyristors on the one hand, and voltage-controlled semiconductor components such as, for example, the unipolar MOS (Metal Oxide Semiconductor) field-effect transistors (MOSFET) or the bipolar MOS-controlled thyristors (MCT) or the MOS-controlled bipolar transistors (IGBT) on the other hand. All the semiconductor components mentioned can only move currents in a current direction (forward direction), i.e. only be switched between an on state and an off state given a specific polarity of the operating voltage present (switchable state). In its off state, each semiconductor component can only block up to a maximum reverse voltage (breakdown voltage). At higher reverse voltages, a charge carrier breakdown takes place, which can rapidly lead to the destruction of the component.
In normal continuous operation (rated operation) of the load, the switching apparatus switches the rated currents required for the electrical load. However, in the event of overloading of the load, for example in the event of switch-on or start-up, there is an increase in the current consumption and thus in the electrical power loss in the load. An overload occurring for an excessively long time jeopardizes the load, for example the motor windings, through overheating. The electric currents flowing through the switching apparatus in such an overload situation may be significantly higher than the rated current and are referred to as overcurrents or overload currents. In the event of motor start-up, by way of example, the start-up currents may assume a value during the start-up time which is up to twelve times higher than the rated current in normal continuous operation.
An even more extreme situation jeopardizing the load and the switching apparatus is a short circuit in the electrical load or in the lines leading to the load, for example, as a result of insulating faults. In the short-circuit situation, very high short-circuit currents occur, which leads very rapidly to thermal damage and usually to the destruction of the load or of parts thereof or of the switching apparatuses themselves.
Consequently, the loads and also, particularly when using semiconductor components, the switching apparatuses must be protected from overcurrents or short-circuit currents flowing over an excessively long period of time. To that end, use is made of special protection apparatuses which, in order to protect the load from excessively high currents, isolate the line branch from the supply network if such a critical current occurs. By way of example, fuses or mechanical protective circuit-breakers with thermal and electromagnetic overcurrent releases can be used as protection apparatuses having such overcurrent or short-circuit current disconnection.
Overload- or short-circuit-proof switching apparatuses with semiconductor components are also known. In this case, it is possible to differentiate between electronic protective circuit-breakers which operate actively with an auxiliary energy and passive, autonomously operating, so-called intrinsically safe electronic protective circuit-breakers.
WO 95/24055 A1 discloses an electrical switching device in which a semiconductor component with two FETs reverse-connected in series and a respective interrupter contact at both sides of the semiconductor component are connected into a line run. The interrupter contacts are switched on or off by a release element connected in parallel with the semiconductor component. A control voltage of a control device is present between gate and source of the two FETs. A current sensor, to which the control device is connected, is arranged in the line run. The control device checks when a permissible short-circuit current is reached or exceeded and then sets the control voltage for the two FETs in such a way that the permissible short-circuit current is not exceeded, by increasing the internal resistance of the FETs by way of the control voltage. The control device generates the control voltage with the aid of an auxiliary energy (extraneous energy). The signal of the current sensor serves only for evaluation of when a short-circuit situation is or is not present.
WO 95/07571 A1 discloses an AC power controller with two silicon-carbide-based MOSFETs, reverse-connected in series. Each SiC-MOSFET can be driven by way of its own gate-source control voltage. According to information in this document, the gate-source voltage is set such that, in the forward direction, it is so large that desired limiting of the drain-source current is established and, in inverse operation, it is only so large that the inner body diodes of the MOSFETs are still currentless. By virtue of the current-limiting property of this circuit, short-circuit currents can be limited to an acceptable level and lowered by way of correspondingly continuously reduced gate-source voltages. The gate-source voltages are generated with the aid of an external energy source.
DE 196 10 135 C1 discloses an intrinsically safe, passively protected or protective electronic switching device having two electrical terminals for applying electrical operating voltages, a silicon-based semiconductor component as switching element and, in addition, a semiconductor arrangement having a high blocking capability as protection element. The protection element includes a first semiconductor region of a predetermined conduction type and at least one further semiconductor region of the opposite conduction type, between which a pn junction is formed in each case. The semiconductor regions are in each case formed with a semiconductor having a breakdown field strength of at least 106 V/cm, in particular diamond, aluminum nitride (AIN), gallium nitride (GaN), indium nitride (InN) and preferably silicon carbide (SiC), in particular the polytypes 3C, 4H and/or 6H.
At least one channel region—adjoining the pn junction—in the first semiconductor region of the protection element is then electrically connected in series with the silicon switching element between the two terminals. The switching element has an on state and an off state given operating voltages of a predetermined polarity. The pn junction of the protection element is electrically connected between the two terminals in the reverse direction for the operating voltages. If the switching element is in its off state, the depletion zone of the at least one pn junction pinches off the channel region in the first semiconductor region of the protection element. As a result, in the off state of the switching element, the largest proportion of the operating voltage between the two terminals is already dropped across the depletion zone of the pn junction. On account of the high breakdown field strength of at least 106 V/cm of the semiconductor provided for the semiconductor regions of the pn junction, the pn junction of the protection element can carry significantly higher reverse voltages than a pn junction formed in silicon and having identical charge carrier concentrations and dimensions. For comparison, the breakdown field strength of silicon is approximately 2·105 V/cm.
The silicon switching element therefore only has to be designed for the remaining part of the reverse voltage between the two terminals. This in turn results in a significantly reduced power loss of the silicon switching element in forward operation. At the pn junction of the protection element, furthermore, in the other circuit branch, the entire operating voltage between the two terminals is present as reverse voltage. In the on state of the silicon component, the channel region in the first semiconductor region of the protection element is opened again and an electric current can then flow between the two terminals through the channel region.
A silicon power MOSFET, preferably of the normally off type, or else a MESFET (Metal Semiconductor Field Effect Transistor) is proposed as the switching element. The protection element is preferably designed as a vertical JFET (Junction Field Effect Transistor). The source of the JFET is short-circuited with the drain of the silicon MOSFET. The drain of the JFET is electrically connected to the second terminal of the electronic switching device. The gate of the JFET is electrically short-circuited with the first terminal of the electronic switching device and the source of the silicon MOSFET. With such a known electronic device, which can be referred to as a hybrid power MOSFET or cascode circuit, it is possible to achieve, in particular, reverse voltages of up to 5000 V and forward currents of between 5 A and 5000 A if silicon carbide (SiC) is used as semiconductor material for the protection element. If, in a further embodiment of the electronic device disclosed in DE 196 10 135 C1, a protection element of an IGBT-like hybrid based on silicon carbide (SiC) is combined with a silicon MOSFET, then it is possible to achieve reverse voltages of up to 10 000 V and rated currents of between 100 A and 1000 A.
The further document DE 198 33 214 C1 discloses a JFET protection element constructed as a mesa structure with epitaxial layers, preferably based on silicon carbide (SiC), as switching element. This JFET protection element having a high blocking capability is proposed particularly for converter applications for variable-speed drives or as AC voltage switches of motor branches in which the switching components have to be operated in “normally off” fashion, i.e. are intended to automatically undergo transition to the blocking state in the event of current failure. To that end, it is proposed to connect the JFET protection element having a high blocking capability in a cascode circuit with a low-voltage MOSFET or low-voltage Smart MOSFET, it being possible to produce the low-voltage FET using known silicon technology.
A so-called intrinsically safe switching device is realized both with the cascode circuit disclosed in DE 196 10 135 C1. Further, in the case of the cascode circuit described in DE 198 33 214 C1, which switching device protects itself and the line branch automatically without an additional auxiliary energy supply and limits short-circuit currents or overload currents to the saturation current of the protection element, in particular the JFET.
In the case of the switching device disclosed in DE 196 10 135 C1 or DE 198 33 214 C1, a problem is posed by the fact that the start-up current or switch-on current required in the event of the start-up operation for an electrical drive or generally in the event of the switch-on of a load may lie in an overload range. What occurs in this case, then, is a situation of a regular overload that is assessable or limited in respect of time or in terms of the current intensity as a normal operating state which can be tolerated in contrast to irregular overload situations with unpredictable duration and overcurrent intensity. In the last-mentioned irregular overload situations, however, the protection element is then intended to limit the current to its saturation current in order to prevent damage to the load.
Therefore, if the protection element is designed in such a way as to achieve overload protection from irregular overloading, then the start-up or switch-on currents required by the load cannot be provided or switched through by the switching device, as a result of which, in turn, proper operation of the load is not possible. If, on the other hand, the protection element is conversely dimensioned such that the saturation current is higher than the start-up current or switch-on current, then the protection element no longer protects the load and the switching element in irregular overload situations in which overcurrents comparable to the start-up currents or switch-on currents flow. The protection component is then to be derated only for the momentary start-up or switch-on operation. For protection from irregular overload situations as well, it is necessary, moreover, to have an additional identification and processing assembly which detects and controls such overload situations separately from the start-up or switch-on situation. However, it would be desirable to use a single protection or switching device to control both the high currents in the event of start-up or switch-on and the current limiting in the event of overload or short-circuit.